Document 1 (JP 2006-60408 A) discloses a network system including a wireless transmitter and wireless receivers. The wireless transmitter transmits a block ACK request signal to each wireless receiver. The block ACK request signal includes a parameter indicating a range of random numbers used for determining a contention window. Each wireless receiver randomly selects a value of the contention window with reference to the parameters included in the block ACK request signal. Each wireless receiver sends a block ACK (response signal) after a lapse of a waiting time period starting from the time of receiving the block ACK request signal. The waiting time period is determined by use of a DIFS (Distributed Inter Frame Space) and the value of the contention window.
In the network system disclosed in Document 1, each wireless receiver randomly determines the value of the contention window. Consequently, it is possible to reduce the probability of collision between the response signals.
In the network system disclosed in Document 1, the maximum value of the contention window is determined irrespective of the number of the wireless receivers. When the number of the wireless receivers is excessive relative to the maximum value of the contention window, the probability of collision between the response signals can not be reduced sufficiently. Therefore, the wireless transmitter may not receive the response signal. In contrast, when the maximum value of the contention window is excessive relative to the number of the wireless receivers, the waiting time period is likely to be prolonged and communication efficiency may become poor.